This invention relates to a method for the uniform application of foamed aqueous mixtures to substrates. It pertains particularly to a method for spreading foamed plywood glues on intermittently-moving wood veneers in the manufacture of plywood and is described herein with particular reference to this application. No limitation thereby is intended, however, since the method is applicable with equal facility to other situations in which foamed liquid is applied to a substrate.
In the U.S. Pat. No. 3,895,984 to Charles N. Cone and Julius M. Steinberg for PLYWOOD MANUFACTURE USING FOAMED GLUES there is disclosed method and apparatus for making plywood by continuously propelling a liquid plywood glue in unfoamed condition at a predetermined flow rate, continuously foaming the glue as it is propelled and continuously extruding or otherwise applying the foamed glue to the surface of wood veneers as they travel along a conveying system in the plywood mill assembly line.
This is accomplished by passing the unfoamed glue through an in-line foamer while maintaining the flow rate of the unfoamed and foamed glue substantially the same on a unit weight of liquiid glue per unit time basis. The veneers to which the foamed glue has been applied then are laid up into plywood assemblies and pressed into plywood panels.
This method has many advantages, including economy of glue use and uniformity of plywood product. In executing the method, however, it is important that the foamed glue be applied uniformly on the plywood veneers. Otherwise, the strength and other properties of the finished plywood will be non-uniform and unpredictable.
Achieving a uniform spread of foamed glue on the surfaces of wood veneers traveling along a plywood production line is a result difficult of achievement. This is for the reason that the operation of the line, and of the mechanical veneer lay-up machines included in it, is intermittent. It is necessary to shut down the line from time to time for various reasons, including mechanical failures, interruption of veneer supply, disarrangement of veneers in the machine, etc. The total down time of the line in a 24-hour period usually amounts to about 12% or a little less than three hours. However, the interruptions are frequent. Each one lasts only a few minutes.
This characteristic of the operation of the plywood production line in turn means that the extruder or other foamed glue spreading device must be timed to stop and start with the line. At first sight, it would appear that this might be accomplished through the agency of a simple control valve actuated by a limit switch or other agency associated with the production line. In fact, it is a matter which is quite difficult of accomplishment.
The control of the flow of foamed glue must be highly accurate. When the line stops, the flow of foamed glue must stop. When the line resumes, the flow of foamed glue must resume and continue at the same rate as when it stopped. Otherwise, the spread rate will change.
Foamed liquids do not share the easily-controlled flow characteristics of ordinary liquids. The rate of flow from the extruder is a function of the pressure within the extruder head. Therefore, in order to insure a constant spread rate, the pressure within the head must be maintained the same at all times.
Simply shutting off the flow to the extruder when the line stops and resuming flow when it starts does not achieve the desired result. Foamed liquids, and particularly foamed phenolic resin glue, have two characteristics that make it necessary to take special precautions if the same flow is to be secured instantaneously after flow cut off as existed before cut off. These are first, a high degree of resistance to flow; and second, a high degree of compressibility.
Because of its high resistance to flow, a considerable amount of pressure is required to make the foamed glue move from the foaming unit where it is produced down the line and into the extruder.
For example, in normal operation of such a unit, the pressure in the line at the discharge port of the foaming unit will be in the neighborhood of 60-70 pounds per square inch. However, because of the high resistance to flow of the foamed liquid, the pressure in the extruder will be only 5-10 pounds per square inch. This steep pressure gradient is maintained during operation of the line by the force exerted by the unfoamed glue being pumped into the foaming unit.
If the foamed discharge from the extruder head is cut off and, simultaneously, the flow of unfoamed glue into the system also is cut off, the pressure gradient will disappear.
This occurrence is explained by the fact that at the instant of cut off, the foam in the line near the foaming unit is more highly compressed than it is in the extruder head. It consequently will expand until the pressure is equalized. This means that after cut off, the pressure inside the extruder head is higher than it was and the pressure in the foaming unit is lower than it was before cut off.
Because of this, when the operation of the line is resumed, the initial rate of flow of foamed glue from the extruder head will be much greater than normal. This then will be followed by a period of diminished rate of flow until the normal pressure gradient has been re-established. This variation in flow in turn will be reflected in uneven application of the glue to the traveling veneers in the plywood production line.
Two solutions to the foregoing problem seem self-evident.
One is to shunt the non-uniform flow of foamed glue away from the extruder after cut off and discard it. This is not feasible first, because it is wasteful and costly; and second, because there is no place to put the discarded glue except in the sewer; and this leads to serious ecology problems.
The other apparent solution is to recycle the non-uniform portion of the glue flow back to the storage vessel which feeds the foamer and combine it there with the raw feed. This expedient is unsatisfactory because mixing the foamed glue with the unfoamed glue changes the density of the latter so that the density of the mixture varies with time. This, in turn, makes non-uniform the amount of glue discharged from the extruder since the amount of foamed glue dispensed by the extruder on a weight basis per unit time is a direct function of the amount of glue pumped to it from the system. The latter amount, in turn, is a direct function of the glue density.
It accordingly is the general purpose of the present invention to provide a method for overcoming the foregoing problem and for applying foamed liquids uniformly to substrates--in particular, foamed aqueous phenolic resin glues to traveling plywood veneers in a plywood mill production line.
It is a further object of the present invention to provide a method of the class described which is simple in concept and construction, easily included in the plywood line, easily and accurately operated and controlled, and relatively maintenance free.
In its broad concept, the present invention comprises a method for the uniform application of foamed liquids to substrates which comprises the following steps:
First, a source of foamable viscous liquid is provided. The viscous liquid is withdrawn from the source and is subjected to a mechanical beating action which tends to reduce the density of the liquid by mechanical entrainment of air therein. The liquid, now of somewhat reduced density, is passed to a reservoir or feed tank from which it is passed to a foamer. The foamed liquid is spread continuously on an intermittently and relatively-moving substrate during the periods of motion of the substrate relative to the foaming means employed.
The foamed liquid is diverted from the substrate during the periods of rest of the latter. The diverted foamed liquid then is subjected to the same mechanical beating action as was applied to the original liquid thus defoaming the liquid and increasing its density. Defoamed liquid is cycled back to the feed tank.
Defoaming the diverted foamed liquid restores it to substantially its original density, i.e., the density of the unfoamed liquid after it has been subjected to the mechanical beating action. As a consequence, the recycled defoamed liquid may be mixed with the mechanically-beaten liquid without adversely affecting its density and, hence, without adversely affecting the amount of liquid on a weight basis spread on the substrate.